Heterogeneous catalysis is widely used in the sphere of chemistry, notably in the petroleum industry or in biomass conversion, which is currently of increasing interest.
Zinc oxide is interesting as a catalytic active phase, most often for its basic properties. It is notably used in large-scale purification methods such as sulfur compound capture or it goes into the catalyst of the Esterfip-H® process developed by IFP, which involves vegetable oil transesterification reactions for biodiesel production.
Zinc oxide ZnO usually has a wurtzite structure, the zinc being surrounded by four oxygen neighbours. Under low dioxygen pressure, the compound becomes oxygen vacant and Zn2+ ions occupy interstitial positions.
Conventionally, the reactivity of alcohols on acidic solids leads to dehydration and therefore to alkenes formation, whereas basic solids favour dehydrogenation reactions leading to aldehydes formation.
Mokwa et al. (Surface Science 117 (1982) 659-667) have studied the decomposition of ethanol under ultra-high vacuum on ZnO monocrystals and concluded that some faces are more active than others. Djega-Mariadassou of al. (J. Chem. Soc., Faraday Trans. 1, 19825, 78, 2447-2454) have studied the influence of the morphology of ZnO on the conversion of isopropanol. Prior to the reaction with isopropanol, the oxide is preheated under vacuum to 300° C. for 12 hours. The conversion kinetics of isopropanol has been studied on zinc oxides of different origins and it has been concluded that the morphology of the ZnO samples has a low impact on the catalytic properties of the dehydrogenation reaction. However, Halawy et al.'s results have shown that the ethanol decomposition reaction exhibits a different selectivity depending on the preparation mode. According to these various studies, it appears that effects related to the nature of the materials can explain reactivity differences. According to Perez-Lopez et al. (Materials Research Bulletin 40 (2005) 2089-2099), the catalytic properties of zinc oxide greatly depend on the size of the crystal, on the atmosphere in which the reaction is carried out since various tests have been conducted in air, hydrogen or nitrogen, and on the zinc salt precursor.
Teams are thus trying to improve the catalytic reactivity of ZnO and more particularly in alcohol conversion processes because of the current interest for biomass products.
The present invention comes within this scope since the inventors have surprisingly discovered that the conversion of alcohols to alkenes and/or oxygen compounds of alcohol type heavier than the initial alcohol, ketones, aldehydes, esters or acids, is markedly improved when the ZnO active catalytic phase is subjected to a thermal pretreatment in an inert and/or reducing atmosphere.